I used SaveFrom.net last week to nick a couple of YouTube videos to my computer, worked fine on my Mac. Not sure about PC though a quick look at the the website seems to indicate both platforms are OK. And it's free
May I know what video you want to download with ? If is from youtube or facebook , then it's simple, try tis http://www.clipconverter.cc/ online video downloader to download youtube hot video and music! It's very helpful! Well...I know it's not right for the law respect...